Petrosia ficiformis (Poiret), one of the Demospongiae Ceractinomorpha, is an oviparous species having relatively large oocytes (140-160 mu m) and a nucleolated nucleus. The perinuclear region is devoid of yolk inclusions and contains an elevated number of dictyosomes and ergastoplasmic lamellae as well as numerous mitochondrial clusters. In comparison the other parts of the cytoplasm are rich in variously organized inclusions. The oocyte is surrounded by archeocyte-type nurse cells. Cell contacts via pseudopods on both cell surfaces, and spherical electron-dense corpuscles in spaces between nurse cells and oocyte have been seen. It is suggested that in Petrosia ficiformis, because of the presence of the nurse cells, the majority of the yolk material is derived from these cells and is then subsequently elaborated. A part of the yolk is also formed through the autosynthetic activity of the oocyte itself.